1. Field of the Invention
This invention relates to steam turbines, and, more particularly, to an improved apparatus and method for utilizing a drain cooler in a steam-to-steam reheat drain system.
2. Description of the Prior Art
Virtually all nuclear steam turbine generators, operating under slightly wet or low superheated initial steam conditions, incorporate steam-to-steam reheat to improve thermal performance and reduce blade erosion. Rising fuel costs in recent years have led to the use of higher initial operating pressures and temperatures and additional reheat features, including an increase in the number of heaters that should be employed in a turbine cycle. The higher pressures and temperatures have led to other design developments including provision for higher outlet water temperatures by utilizing superheat of the steam, and drain cooling sections in the heaters that subcool condensate.
Current practice with respect to steam-to-steam reheater drains is to discharge the drain fluid, a mixture of condensed steam and scavenging steam, from the high pressure reheater in a moisture-separator-reheater (hereinafter MSR) to the highest pressure feedwater heater where the fluid is combined with the condensed heater steam from the first turbine extraction point. "Scavenging steam" refers to small amounts of dry steam bled from the main steam supply lines and directed through the tubes of the reheater bundle to prevent the condensate from subcooling and collecting, particularly in those tubes at the lower elevations of the bundle or the outermost U-shaped tubes of the bundle which are exposed to the lowest temperature incoming steam to be reheated. Condensate collection may result in subcooling and the associated sudden temperature change may damage piping when condensate is eventually blown from the piping by the pressure build-up. Steam-to-steam reheat designs usually employ approximately 2% of total reheater steam supply at rated load for scavenging steam to prevent moisture build-up in the reheater tubes.
From the highest pressure feedwater heater, the condensed steam and other drain flows are then discharged or cascaded seriatim to lower and lower pressure feedwater heaters until at some point in the cycle, the flows become part of the main feedwater stream.
As previously disclosed in U.S. patent application Ser. No. (53,980) filed by Silvestri and Viscovich and assigned to Westinghouse Electric Corporation, the drains leaving the MSR high pressure reheater are considerably hotter than the feedwater leaving the highest pressure feedwater heater, as much as 55.degree. C. (100.degree. F.) at rated load, and in excess of 140.degree. C. (250.degree. F.) at 25% load. Accordingly, the drains must be throttled down to the feedwater pressure prior to heat exchange. This results in a loss in thermal efficiency.
One suggested method of minimizing this loss is to pump the high pressure reheater drain fluid into the outlet of the highest pressure feedwater heater. Major drawbacks of this method are: (a) an additional pump is required; (b) the difficulty of avoiding cavitation due either to insufficient net positive suction head in steady state conditions or to flashing during transients; and (c) disposal of scavenging steam that is used to enhance the reheater tube bundle reliability.
The invention of the above-referenced application, which is incoporated herein by reference, provides a method and apparatus for improving the thermal efficiency of steam-to-steam reheating systems within steam turbine generator systems. It allows the reheater drain fluid to be directly added to the feedwater stream without the need for additional pumping, by using a drain cooler to receive the high pressure reheater drain fluid which passes the drain fluid in heat exchange relationship with condensate from the discharge of the highest pressure feedwater heater. This avoids the loss of thermal efficiency resulting from throttling of the reheater drain pressure. Heat rate improvement is greater when the system is operated at less than 100% load. The referenced invention was designed for field retrofit application to single and multi-stage moisture-separator-reheaters. These existing systems include drain receivers with level controls. Fluid from high pressure reheater drains is collected in the drain receivers and then directed to a heat exchanger (drain cooler) in heat exchange relationship with condensate from a high pressure feedwater heater. The use of a drain cooler avoids loss of thermal efficiency from throttling of reheater drain pressure.
There is a corresponding need to apply the drain cooler concept to new as well as retrofit installations. Further, there always exists the need for additional improvements in the thermal efficiency of steam generation systems while avoiding operational and maintenance problems. It is therefore a principal object of this invention to improve and enhance the drain cooler concept.
Conventional reheater drain systems customarily employ a pressure breakdown section between the MSR reheater drain connection and the feedwater heater receiving the drain fluid, and a level-controlled drain receiver to accept the condensed heating steam. There is a significant reliability problem with drain receivers, which frequently produce internal flooding in the drain tube bundle from the high pressure MSR. Such flooding has contributed to numerous damaged tube bundles, necessitating reduced load operation at impaired plant efficiency.
Further, because of the decrease in heater pressure at low loads, accompanied in many instances with an increase in reheater supply pressure, the percentage of scavenging steam increases with decreasing load. An increase in scavenging steam has a small affect on the heat rate of a cycle employing a drain cooler, as shown in Tables I and II of U.S. patent application Ser. No. (53,980).
In the absence of other control means, the amount of scavenging steam is controlled by the condensing capability of the drain cooler. Calculations for two sample plants employing the drain cooler concept, a single stage and a two stage reheat design, reveal that if the drain cooler were sized to accept 2% scavenging steam at 50% load, the scavenging steam at rated load would be in the 4.2% range for a single stage design (Table I) or 5.4% range for a two stage design (Table II). The percentage of scavenging steam would decrease as load is reduced and would remain at about 2% when operating below 50% load.
It can be seen from the third and fourth columns of each of these Tables that with rated load scavenging steam flow above 2%, there is some reduction in heat rate improvement, 1 BTU/KWH (1.055 KJ/KWH) for a two stage reheat design and 2 BTU/KWH (2.110 KJ/KWH) for a two stage design. Accordingly, it is an object of the present invention to improve heat rates at lower loads, i.e., eliminate such reduction in heat rate improvement, by providing means for optimizing the amount of scavenging steam for lower operating loads. This object is accomplished with means for adjusting the drain cooler heat transfer capability to keep the scavenging steam at 2% at all loads.
Another object of this invention is to eliminate the potential internal flooding of the bundle drains, while at the same time incorporating the drain cooler concept, to reduce pressure drops in the drain piping between the high pressure reheater bundle drain connection and terminal point at the shell side inlet of the drain cooler.